The configuration of a conventional speaker is described with reference to its sectional view in FIG. 7. A magnetic circuit 15 includes a magnet 15a, lower plate 15b, and upper plate 15c. 
A frame 16 is bonded to the magnetic circuit 15. A damper 17 holds a voice coil 18. The outer circumference of the damper 17 is bonded to the frame 16, and its inner circumference is bonded to the voice coil 18 whose coil 18a is embedded in the magnetic gap 15d of the magnetic circuit 15.
A speaker diaphragm 19 is bonded to the frame 16 via an edge 19a bonded to its outer circumference, and the inner circumference of the speaker diaphragm 19 is bonded to the voice coil 18. This speaker diaphragm 19 is generally made mainly of paper or thin resin plates, which is selected depending on the need for weather resistance and required acoustic characteristics.
A method for manufacturing a typical speaker diaphragm 19 made of resin, more specifically a polyolefin polyethylene speaker diaphragm is described next.
A speaker diaphragm made of polyethylene, which is a type of polyolefin system, has low material density which gives the speaker diaphragm a low mass. It also has relatively large internal loss with respect to mechanical vibration, which improves the frequency characteristic of the speaker. Accordingly, polyethylene speaker diaphragms are commonly used in speakers. However, a polyethylene speaker diaphragm has low adhesivity, making it essential to activate the surface of the speaker diaphragm to improve bonding strength.
Common conventional methods for activating the surface of the speaker diaphragm 19 include the application of primer after corona discharge, and surface treatment of the speaker diaphragm 19 by the gas plasma treatment method using parallel flat electrodes 20 and 21 as shown in FIGS. 8 and 9.
However, the conventional surface activating technology for treating the surface of the speaker diaphragm has the following disadvantages.
As for the method involving surface activation by corona discharge and primer application, a large processing apparatus is required because only the areas close to electrodes 20 and 21 are activated if the electrodes are small, causing a deviation in wettability of the speaker diaphragm 19. In addition this treatment takes about 30 seconds for one face of the speaker diaphragm 19. The workpiece then needs to be flipped or the speaker diaphragm 19 needs to be flipped to apply treatment to the other face, requiring more than one minute for each piece and seriously degrading productivity.
Furthermore, it has another problem that the speaker diaphragm may deform during corona discharge due to high temperatures above 80° C. in the reactive chamber.
The method using the parallel flat electrode 20 in FIG. 8 also creates the risk of heat deformation and low productivity of the speaker diaphragm 19. The method using the parallel flat electrode 21 in FIG. 9 may also cause low productivity. If more, than one speaker diaphragm 19 is handled at once to solve the problem of low productivity, it may still have the risk of heat deformation, and significant difference in wettability between the periphery and the center.
The present invention aims to solve these disadvantages. By offering a method for manufacturing a speaker diaphragm assuring stable quality and high productivity, the present invention offers a speaker diaphragm with stable quality and a speaker employing such diaphragm.